Autoimmune diseases affect up to 5% of the population. They stem from alterations in immune regulation that lead to a breakdown of self-tolerance to tissue antigens. The goal of these studies is to develop a preclinical strategy, based on tolerance induction, that will be applicable to the treatment of human autoimmune disease. Toward this goal, we have established a preclinical model for studying tolerance and regulatory T cells in nonhuman primates (NHP). Although initially developed as a means to induce stable transplantation tolerance, the model allows us to explore basic questions about T cell tolerance in NHP. We hypothesize that our highly effective tolerance induction strategy allows the T cell component of the immune system to reset in the presence of antigen and IL-10, providing a window of opportunity to drive Tregs and establish stable tolerance. We propose that this unique therapeutic approach in NHP will translate to human autoimmune disease. The proposed studies will examine homeostatic T cell expansion in vivo in the context of IL-10 and Tregs to determine which regulatory elements are central to the maintenance of NHP tolerance to tissue antigens. The specific aims are as follows: Aim 1: To determine T cell regulatory mechanisms during the restoration of T cell homeostasis and induction of immune tolerance following anti-CD3 immunotoxin (IT) plus Deoxyspergualin (DSG) therapy in adult rhesus (RH) macaques. Aim 2: To examine the use of recombinant IL-10 therapy as a supplement to IT plus DSG treatment to enhance the consistency of activating T regulatory cells and obtaining stable tolerance. Aim 3: To determine the effect of IT plus DSG treatment on T cell memory. The studies are original in that, to our knowledge, there is no published information on CD3+ NKT, CD4+CD25+ and CD8+CD28- regulatory cell populations in NHP. This situation persists, despite the importance of NHP as the penultimate preclinical model for examination of tolerance and vaccine interventions. The proposed studies are expected to yield new information to optimize the achievement of tolerance and to modify immunologic memory. These questions - the elements that regulate tolerance during homeostatic repopulation, the role of antigen, and the effect of these on immunologic memory - are the focus of this proposal.